1. Field of the Invention
The present invention relates generally to a mobile communication system. In particular, the present invention relates to an apparatus and method for transmitting/receiving data using a multiple antenna diversity scheme.
2. Description of the Related Art
Mobile communication systems have evolved from voice communication systems to packet service communication systems. The packet service communication systems transmit burst packet data to multiple mobile stations. They are designed for large-volume data transmission. These packet service communication systems have been developed for high-speed packet service. To provide the high-speed packet service, a standardization working group for asynchronous systems, 3rd Generation Partnership Project (3GPP) proposed High Speed Downlink Packet Access (HSDPA), whereas a standardization working group for synchronous systems, 3rd Generation Partnership Project2 (3GPP2) proposed 1× Evolution Data Only (1×EV-DO) and 1× Evolution Data and Voice (1×EV-DV). HSDPA, 1×EV-DO and 1×EV-DV all support high-speed packet transmission to ensure reliable internet service such as the Web. For high-speed packet service, both circuit data transmission such as voice service and packet data transmission are actively provided by optimizing peak throughput as well as average throughput.
HSDPA will now be described. HSDPA is a generic term referring to devices, systems and methods using the High Speed-Downlink Shared CHannel (HS-DSCH) for supporting downlink packet data transmission at a high rate and its related control channels in Wideband-Code Division Multiple Access (W-CDMA). HSDPA, which was proposed by the 3GPP and adopted as the standard for 3rd generation asynchronous mobile communication systems, will be described by way of example.
Three techniques were introduced into the HSDPA communication system to support high-speed packet data transmission: Adaptive Modulation and Coding (AMC), Hybrid Automatic Retransmission Request (HARQ), and Fast Cell Select (FCS).
AMC: A modulation scheme and a coding method are selected for a data channel according to the channel condition between a cell, namely a Node B and a User Equipment (UE), to thereby increase the bandwidth efficiency of the entire cell. Modulation schemes and codings are used in combination. Each modulation and coding combination is termed a Modulation and Coding Scheme (MCS). MCSs can be labeled from level 1 to level N. A data channel signal is modulated and encoded by an MCS chosen adaptively according to the channel condition between the UE and its communicating Node B. Thus, the system efficiency of the Node B is increased.
HARQ, especially N-channel Stop And Wait HARQ (SAW HARQ): In accordance with a typical Automatic Retransmission Request (ARQ), Acknowledgement (ACK) signals and retransmission packet data are exchanged between a UE and an Radio Network Controller (RNC). Meanwhile, the HARQ scheme adopts the following two novel procedures to increase ARQ transmission efficiency. One is to exchange a retransmission request and its related response between a UE and a Node B, and the other is to temporarily store bad data and combine the stored data with a retransmitted version of the data. In the HSDPA communication system, ACK signals and retransmission packet data are exchanged between the UE and the MAC HS-DSCH of the Node B, and the N-channel SAW HARQ establishes N logical channels and transmits a plurality of packets without receiving an ACK signal for a previously transmitted packet. As compared to the N-channel SAW HARQ, SAW ARQ requires reception of an ACK signal for a previously transmitted packet data in order to transmit the next packet data. Thus the ACK signal for the previous packet must be received despite the capability of transmitting the current packet data. On the contrary, the N-channel SAW HARQ allows transmission of successive packets without receiving the ACK signal for the previously transmitted packet data, resulting in an increase in the channel use efficiency. That is, N logical channels, which can be identified by times or channel numbers assigned to them, are established between the UE and the Node B, so that the UE can determine the channel that has delivered a received packet and take appropriate measures such as the rearrangement of packets in the right order or soft combing of corresponding packet data.
FCS: When a UE supporting HSDPA is positioned in a soft handover region, it quickly selects a cell with a good channel condition. Specifically, if the UE enters a soft handover region between a first Node B and a second Node B, it establishes radio links with a plurality of Node Bs. A set of Node Bs with which the radio links are established are called an active set. The UE receives HSDPA packet data only from the cell with the best channel condition, thus reducing whole interference. The UE also monitors channels from the active Node Bs periodically. In the presence of a cell with a better channel condition than the current best cell, the UE transmits a best cell indicator (BCI) to all the active Node Bs to substitute the new best cell for the old best cell. The BCI includes the ID of the new best cell. The active Node Bs check the cell ID included in the received BCI and only the new best cell transmits packet data to the UE on the HS-DSCH.
As described above, many novel techniques were proposed in order to increase the data rate in the HSDPA communication system. Like HSDPA, 1×EV-DO and 1×EV-DV were designed to increase data rates. Hence, increasing data rates is a challenging issue. Aside from AMC, HARQ, and FCS, a multiple antenna scheme is used to increase data rates. Since the multiple antenna scheme works in the space domain, it overcomes the problem of limited bandwidth resources in the frequency domain.
The multiple antenna diversity scheme will now described.
In a radio channel environment, a mobile communication system suffers signal distortion because of various factors such as multi-path interference, shadowing, propagation attenuation, time-varying noise, and interference. Fading caused by the multi-path interference is closely associated with reflective objects or the mobility of a user, that is, the mobility of a UE. The fading results in mixed reception of an actual transmission signal and an interference signal. The received signal has serious distortion, which degrades the whole mobile communication system performance. Fading is a serious obstacle to high-speed data communication in a radio channel environment in that the fading results in distortion in the amplitude and phase of the received signal. Many studies are being conducted to overcome fading. Thus, the mobile communication system must minimize the loss inherent to radio channels, such as fading, and user interference in order to transmit data at high rates. Diversity is used as a solution to fading. Amongst diversity schemes, space diversity uses multiple antennas.
Transmit antenna diversity has emerged as an effective way to combat fading. The transmit antenna diversity scheme receives a plurality of transmission signals experiencing independent fading under the radio channel environment and copes with fading-caused distortion. The transmit antenna diversity is classified into time diversity, frequency diversity, multi-path diversity, and space diversity. In other words, the mobile communication system must overcome fading that seriously influences communication performance in order to carry out high-speed data transmission reliably. Fading reduces the amplitude of a received signal by several decibels to tens of decibels. Hence, the above-described diversity schemes are adopted to combat fading. For example, a Code Division Multiple Access (CDMA) communication system uses a rake receiver for implementing diversity reception based on the delay spread of a channel. The rake receiver provides diversity gain for received multi-path signals. However, if a channel delay spread is relatively small, the rake receiver cannot offer a desired diversity gain.
Time diversity effectively copes with burst errors generated in the radio channel environment by use of interleaving and coding. Typically, the time diversity applies to a Doppler spread channel. A distinctive shortcoming of the time diversity is that it is difficult to achieve the diversity effect from a slow-fading Doppler channel.
Space diversity is used for a channel having a small delay spread, for example, an indoor channel and a pedestrian channel being a slow fading Doppler channel. The space diversity scheme achieves diversity gain by use of two or more antennas. If a signal transmitted through one antenna is attenuated by fading, diversity gain is obtained by receiving signals transmitted through the other antennas. The space diversity is branched into receive antenna diversity using a plurality of receive antennas and transmit antenna diversity using a plurality of transmit antennas. Considering the difficulty in adopting receive diversity for a UE in terms of hardware miniaturization or manufacturing cost, a transmit antenna is recommended for a Node B.
Frequency diversity achieves diversity gain from signals transmitted with different frequencies and propagated in different paths. In this multi-path diversity scheme, the multi-path signals have different fading information. Therefore, diversity is obtained by separating the multi-path signals.
To solve the above-described problems, methods have been proposed in which the same diversity gain is obtained as if a plurality of receive antennas were used to combat fading on a radio channel e.g., a Node B transmits a signal through a plurality of transmit antennas and a UE receives the signal through one or two receive antennas. Techniques of implementing space diversity using two or more transmit antennas are attracting much interest for the future generation mobile communication system proposed by the 3GPP. Such a transmit antenna diversity scheme can be implemented in a closed loop or an open loop.
The closed loop transmit antenna diversity differs from the open loop in that a UE provides back downlink channel information to a Node B in the former, while the feedback information is not required in the latter. Space Time Transmit Diversity (STTD), a kind of space diversity, is a major open-loop transmit antenna diversity technique. In STTD, space-time coding is used instead of information about radio channel status.
A major closed-loop transmit antenna diversity scheme is Transmit Antenna Array (TxAA) that uses feedback information about radio channel status received from a UE. While the present invention is applicable to all mobile communication systems adopting diversity schemes using multiple antennas and HARQ as well as open-loop transmit diversity and closed-loop transmit diversity, it will be described in the context of the open-loop transmit diversity.
With reference to FIG. 1, the structure of a transmitter in a mobile communication system using the multiple antenna diversity scheme will be described.
FIG. 1 is a block diagram of a transmitter in a mobile communication system using a conventional multiple antenna diversity scheme.
It is assumed that the exemplary transmitter described herein supports Double Space Time Transmit Diversity (DSTTD). DSTTD is an extension of STTD. It offers a higher space diversity gain than STTD. Referring to FIG. 1, the transmitter typically comprises a Cyclic Redundancy Check (CRC) adder 111, a turbo encoder 113, a rate matcher 115, an interleaver 117, a modulator 119, a serial-to-parallel converter (SPC) 121, STTD encoders 123 and 125, a plurality of spreaders 131, 141, 151 and 161, a plurality of transmit antennas 133, 143, 153 and 163, and an Adaptive Modulation and Coding Scheme (AMCS) controller 150.
For the input of information bits, the CRC adder 111 adds a CRC to the information bits, for an error check. The turbo encoder 113 turbo-encodes the signal received from the CRC adder 111. As can be appreciated, many acceptable coding schemes may be used in conventional devices, including, for example, convolutional encoding. The turbo encoder 113 encodes at a predetermined coding rate. The ratio between systematic bits and parity bits output from the turbo encoder 113 is determined according to the coding rate. For example, if the coding rate is a symmetric coding rate, ½ (r=½), the turbo encoder 113 outputs one systematic bit and one parity bit for the input of one bit. For another example, if the coding rate is an asymmetric coding rate, ¾ (r=¾), the turbo encoder 113 outputs three systematic bits and one parity bit for the input of three bits. Of course any acceptable coding rate may be employed. The rate matcher 115 punctures or repeats the coded bits received from the turbo encoder according to the transmission capacity of an actual physical channel in the mobile communication system. The interleaver 117 interleaves the output of the rate matcher 115 in a predetermined interleaving manner to prevent burst errors. The modulator 119 modulates the interleaved bits using a predetermined modulation scheme such as Binary Phase Shift Keying (BPSK), Quadrature Phase Shift Keying (QPSK), or 16 Quadrature Amplitude Modulation (16QAM). In 16QAM, four coded bits are mapped to one modulation symbol. In QPSK, two coded bits are mapped to one modulation symbol.
The SPC 121 converts a serial modulation symbol sequence to parallel symbol sequences. Specifically, the SPC 121 pairs every two modulation symbols received from the modulator 119 and provides every two modulation symbol pairs separately to the STTD encoders 123 and 125. The operations of the SPC 121 and the STTD encoders 123 and 125 will be detailed later. The STTD encoder 123 encodes the received modulation symbol pair in STTD and transmits the coded symbols to both the spreaders 131 and 141. The spreader 131 spreads the received symbols with a predetermined spreading code and transmits the spread symbols through the transmit antenna 133. The spreader 141 spreads the received symbols with a predetermined spreading code and transmits the spread symbols through the transmit antenna 143. The STTD encoder 125 encodes the received modulation symbol pair in STTD and transmits the coded symbols to both the spreaders 151 and 161. The spreader 151 spreads the received symbols with a predetermined spreading code and transmits the spread symbols through the transmit antenna 153. The spreader 161 spreads the received symbols with a predetermined spreading code and transmits the spread symbols through the transmit antenna 163.
The AMC controller 150 controls the operations of the rate matcher 115 and the modulator 119. The AMCS controller 150 determines a coding rate and a modulation scheme for the transmitter by considering the present radio channel status. For example, if the present radio channel status is relatively good, the AMCS controller 150 selects a higher coding rate and a higher-order modulation scheme, whereas if the present radio channel status is relatively bad, the AMCS controller 150 selects a lower coding rate and a lower-order modulation scheme. As stated earlier, because the coding rate of the turbo encoder 113 is fixed, the AMCS controller 150 controls the rate matcher 115 to match the number of the coded bits from the turbo encoder 113 to a coding rate determined in the AMCS controller 150. Also, the AMCS controller 150 controls the modulator 119 according to the selected modulation scheme. The AMCS controller 150 is aware of the radio channel status via signaling from a higher layer. In addition, the AMCS controller 150 controls data retransmission.
A detailed description will now be made below of the operations of the SPC 121 and the STTD encoders 123 and 125.
Although not shown, the four modulation symbols from the modulator 119 can be denoted as “S1, S2, S3 and S4”. The SPC 121 parallelizes S1, S2, S3 and S4 and provides S1 and S2 to the STTD encoder 123 and S3 and S4 to the STTD encoder 125. The STTD encoders 123 and 125 encode S1 & S2 and S3 & S4 in STTD. After the STTD encoding, (S1, S2) are transmitted in the form of (S1, S2) and (−S2*, S1*) as illustrated in Table 1 below.
TABLE 1tt + TTransmit antenna 133S1S2Transmit antenna 143−S2* S1*
As noted in Table 1, using STTD encoding, the STTD encoder 123 transmits S1 through the transmit antenna 133 and −S2* through the transmit antenna 143 at time t, and transmits S2 through the transmit antenna 133 and S1* through the transmit antenna 143 at time (t+T).
After the STTD encoding, (S3, S4) are transmitted in the form of (S3, S4) and (−S4*, S3*) as illustrated in Table 2 below.
TABLE 2tt + TTransmit antenna 153S3S4Transmit antenna 163−S4* S3*
As noted in Table 2, using STTD encoding, the STTD encoder 125 transmits S3 through the transmit antenna 153 and −S4* through the transmit antenna 163 at time t, and transmits S4 through the transmit antenna 153 and S3* through the transmit antenna 163 at time (t+T). The structure of the transmitter in the mobile communication system using the multiple antenna diversity scheme has been described above in connection with FIG. 1. Now, the structure of a receiver in the mobile communication system using the multiple antenna diversity scheme will be described with reference to FIG. 2.
FIG. 2 is a block diagram of a receiver in the mobile communication system using a conventional multiple antenna diversity scheme.
The receiver illustrated in FIG. 2 is configured in relation to the transmitter illustrated in FIG. 1 so that it can receive DSTTD signals from the transmitter. Referring to FIG. 2, the exemplary conventional receiver comprises a plurality of receive antennas 211, 221, 231 and 241, a plurality of despreaders 213, 223, 233 and 243, STTD decoders 251 and 253, a parallel-to-serial converter (PSC) 255, a demodulator 257, a deinterleaver 259, a de-rate matcher 261, a combiner 263, a turbo decoder 265, and a CRC checker 267.
Signals received at the receive antennas 211, 221, 231 and 241 are applied to the input of the despreaders 213, 223, 233 and 243, respectively. That is, a signal received through the receive antenna 211 is provided to the despreader 213, a signal received through the receive antenna 221 is provided to the despreader 223, a signal received through the receive antenna 231 is provided to the despreader 233, and a signal received through the receive antenna 241 is provided to the despreader 243. The despreaders 213, 223, 233 and 243 despread their received signals using the spreading codes in the spreaders of the transmitter.
The STTD decoder 251 decodes the signals received from the despreaders 213 and 223 in STTD, and the STTD decoder 253 decodes the signals received from the despreaders 233 and 243 in STTD. These operations in the STTD decoders 251 and 253 will be detailed below.
Because the four STTD-encoded symbols illustrated in Table 1 and Table 2 are transmitted through the four transmit antennas as described with reference to FIG. 1, signals received at the receiver are expressed asr1=h1s1+h2s2+h3s3+h4s4+n1 r2=−h1s2*+h2s1*−h3s4*+h4s3*+n2  (1)where r1 and r2 are signals received at corresponding time points, h1, h2, h3 and h4 are channel responses from the four respective transmit antennas, and n1 and n2 are Additive White Gaussian Noise (AWGN).
The STTD decoder 251 decodes the received signals expressed as Eq. (1) in STTD and outputs signals expressed as{tilde over (s)}1=h1*r1+h2r2*{tilde over (s)}2=h2*r1−h1r2*  (2)and the STTD decoder 253 decodes the received signals expressed as Eq. (1) in STTD and outputs signals expressed as{tilde over (s)}3=h3*r3+h4r4*{tilde over (s)}4=h4*r3−h3r4*  (3)
Eq. (1) and Eq. (2) are developed to{tilde over (s)}1=(α12+α22)s1+n1′{tilde over (s)}2=(α12+α22)s2+n2′{tilde over (s)}3=(α32+α42)s3+n3′{tilde over (s)}4=(α32+α42)s4+n4′  (4)where n is a noise component and αj is a diversity gain obtained from a jth transmit antenna.
It is noted in Eq. (4) that signal components are separated from noise components and four signal components having diversity gains, {tilde over (s)}1, {tilde over (s)}2, {tilde over (s)}3, {tilde over (s)}4 are received.
Consequently, the STTD decoder 251 outputs {tilde over (s)}1 and {tilde over (s)}2 and the STTD decoder 253 outputs {tilde over (s)}3 and {tilde over (s)}4. Therefore, the PSC 255 serializes the signals received from the STTD decoders 251 and 253 and outputs {tilde over (s)}1, {tilde over (s)}2, {tilde over (s)}3, {tilde over (s)}4 to the demodulator 257. The demodulator 257 demodulates the received signal in relation to the modulation scheme used in the transmitter. The deinterleaver 259 deinterleaves the demodulated bits in correspondence with the interleaving method used in the transmitter. The de-rate matcher 261 de-rate matches the deinterleaved bits in relation to the rate matching performed in the transmitter.
The combiner 263 combines the present coded bits received from the de-rate matcher 261 with an already stored version of the coded bits. This means that data initially transmitted from the transmitter had errors and was not received normally. Therefore, if the receiver requests a retransmission and the transmitter retransmits the data, the retransmitted data is combined with the previous defective data in the combiner 263. If the present data is initial transmission data, the coded bits bypass the combiner 263 without being combined. The turbo decoder 265 turbo-decodes the signal received from the combiner 263 in relation to the turbo encoding performed in the transmitter. The turbo decoder 265 receives coded bits, that is, systematic bits and parity bits from the combiner 263 and decodes the systematic bits. The CRC checker 267 extracts CRC bits from the systematic bits (i.e. information bits) received from the turbo decoder 265 on a packet basis and determines whether the packet has errors based on the CRC bits. Determining that no errors occurred in the packet, the CRC checker 267 outputs the packet and transmits an ACK signal for the packet to the transmitter, indicating normal reception of the packet. On the contrary, if the CRC check result indicates an occurrence of errors in the packet, the CRC checker 267 transmits an NACK signal to the transmitter, requesting a retransmission of the defective packet.
If the CRC checker 267 outputs the ACK signal, a buffer in the combiner 263 is initialized, while coded bits stored in the buffer are deleted. On the other hand, if the CRC checker 267 outputs the NACK signal, the coded bits remain in the buffer of the combiner 263.
In case of data retransmission, S1 and S2 offer a diversity gain by α1α2 and S3 and S4 offer a diversity gain by α3α4. Because the transmitter transmits the same symbols through the same transmit antennas at an initial transmission and a retransmission, the receiver receives the symbols on channels in the same channel status at the initial transmission and retransmission. For example, if S1 and S2 are delivered on a channel in an error or distorted state and thus not received normally, the receiver requests a retransmission of S1 and S2. Then, the transmitter retransmits S1 and S2 through the same transmit antennas, that is, on the same channels. Hence, S1 and S2 are delivered again in error or distorted state, increasing an error probability. In this case, only part of whatever diversity that can be achieved is available, leading to the decrease in resources and efficiency.